1. Field of the Invention
The present invention relates to a trimming capacitor and more particularly to a laminated ceramic trimming capacitor.
2. Description of the Related Art
A trimming capacitor is used when electrostatic capacitances of various values are required. For instance, in the example shown in FIGS. 15-20, the trimming capacitor is formed as a laminate 86 by laminating a dielectric sheet 82 on which internal capacitor electrodes 84 and 85 are provided, a dielectric sheet 82 on which a trimming capacitor electrode 83 is provided and a dielectric sheet 82 having no electrode and by sintering them in a body. The laminate 86 is also provided with external electrodes 90 and 91 at both ends thereof as shown in FIG. 16.
Referring to FIG. 17, electrostatic capacitance is mainly generated at parts 87a and 87b (shown by dotted slant lines) where the trimming capacitor electrode 83 overlaps with the internal capacitor electrodes 84 and 85. The electrostatic capacitance generated at the part 87a is represented as C1 and the electrostatic capacitance generated at the part 87b as C2 in this case. The total electrostatic capacitance formed in the trimming capacitor 81 turns out to be a serial connection of C1 and C2 and is expressed by (C1.times.C2)/(C1+C2).
To adjust the capacitance, part of the trimming capacitor electrode 83 of this trimming capacitor 81 is normally removed by a laser beam L as shown in FIG. 18, to reduce an area thereof facing to the internal capacitor electrode 85 (or the internal capacitor electrode 84) set the electrostatic capacitance to a desired value.
However, in the prior art trimming capacitor 81, the rate of decrease of electrostatic capacitance in response to such a laser trimming operation is not constant, but rather it changes depending on the trimming direction of the laser beam L applied to the trimming capacitor electrode 83. That is, the rate of decrease of the electrostatic capacitance, when the laser beam L is scanned in the direction to remove an edge 83a on the short side of the trimming capacitor electrode 83, is not the same as when the laser beam L is scanned in the X direction to remove an edge 83b on the long side of the trimming capacitor electrode 83 as shown in FIG. 19.
When the edge 83a is removed as shown in FIG. 17, the rate of decrease of the total electrostatic capacitance changes non-linearly as indicated by a solid line 95 in FIG. 20 because only one of the electrostatic capacitances C1 and C2 decreases linearly corresponding to the trimming length (Y direction) and the other will not change. When the edge 83b is removed on the other hand, the rate of decrease of the total electrostatic capacitance changes linearly as indicated by a solid line 96 in FIG. 20 because both electrostatic capacitances C1 and C2 change linearly corresponding to the trimming length (X direction).
Further, unless the length l1 is equal to the length 12 shown in FIG. 17, the trimming length to obtain an electrostatic capacitance of zero is different in the X and Y directions.
From the above reasons, it has been necessary to set the trimming direction per parts corresponding to characteristics of the trimming capacitor 81 and to align the trimming capacitor 81 in the correct direction in accordance to that in trimming the prior art trimming capacitor 81.
Accordingly, it would be advantageous to provide a trimming capacitor which can be trimmed the same length in either direction to obtain a given change capacitance.